1. Field of the Invention
The present invention relates to a variable power optical system and an imaging device used for a video camera, an electronic still camera and so forth. More particularly, the present invention relates to a variable power optical system and an imaging device equipped with the variable power optical system capable of being preferably used as a surveillance camera.
2. Description of Related Art
There is an increasing demand for surveillance cameras used when empty facilities are operated. Recently, surveillance cameras, the magnifying power of which can be varied, are demanded. A variable power optical system used for the above surveillance cameras needs to be a bright optical system, the aperture ratio of which is high so that a photographic object can be specified even under the photographing condition of low illuminance. Further, the optical system applied to the surveillance cameras needs to be compact and provided with excellent optical performance.
An example of the variable power optical system is disclosed in JP-A-2006-119574, in which the aperture ratio is high, but the size is small and the optical performance is excellent, and which is capable of being mounted on a surveillance camera. The optical system described in JP-A-2006-119574 includes: a first group which is negative; a stop; and a second group which is positive, in order from the object side. The second group includes four lenses including two biconvex lenses, a negative meniscus lens and a biconvex lens in order from the object side.
In this connection, recently, there is a rising demand for an image of high quality even in the use of surveillance cameras. Especially, a demand for a variable power optical system is increasing which is applicable to cameras having imaging elements, the number of pixels of which is not less than one million. However, in order to satisfy the above demand by the conventional optical system, it is necessary to reduce an aperture ratio. In this case, it is difficult to use this optical system under an imaging condition of the use of surveillance cameras in which illuminance is low.